The present invention relates to a submersible electric motor for use in a submersible pump or the like, and more particularly to a thrust bearing for supporting an upward thrust given to a shaft of the electric motor.
In a conventional submersible pump comprising a submersible electric motor arranged in its lower part and a pump unit arranged thereon in its upper part, a shaft of the electric motor extends upward into a pump casing of the pump unit and a pump impeller and a case ring are mounted to the shaft. The motor shaft is supported by upper and lower thrust bearings. A downward thrust load is usually supported by the lower bearing, and the upper thrust bearing for supporing an upward thrust load restricts the upward movement of the shaft when an upward thrust is given to the shaft.
The thrust loads of the upper and the lower bearings may be properly determined. For instance, an operating point of a pump is selected so that the lower and the upper thrust loads may be one ton (1000 kg.) and one tenth thereof i.e., 100 kg., respectively.
Therefore, the upper thrust bearing supporting the upward thrust load may be a solid type having an anular continuous slide surface such as an annular ring type and a collar having a flange type. Further, usually, the slide contact member of the upper thrust bearing does not contact the slide contact member of the shaft, that is, there is a space between the two contact surfaces of the upper thrust bearing and the shaft. When the thrust load is changed from the downward thrust to the upward thrust, the shaft is raised by the upward thrust, and then the slide contact member of the shaft contacts the slide contact member of the upper thrust bearing.
In the above described conventional upright submersible pump, on starting or operating under an excessive discharge amount, an upward thrust load is applied to the shaft. Accordingly, the operating point of the pump is restricted by the load capacity of the upper thrust bearing supporting the upward thrust load; and the discharge volume is adjusted by using a discharge valve. The upward thrust load on starting gives a shock to the upper thrust bearing, and thus a flange-form solid type thrust bearing is used. However, this type of thrust bearing has a disadvantage in that it has a small load capacity for supporting the upward thrust load during the continuous operation under the excessive discharge amount, and further, it is difficult to obtain a large bearing capacity. Hence, a floating type the bearing such as the Michell thrust bearing or the Kingsbury thrust bearing, and the like, having a large load capacity can be utilized as the upper thrust bearing. However, in such a bearing, there is a clearance 0.5-1.0 mm between thrust pads of an upper thrust bearing and a slide contact member made of a carbon material or a metallic material, which is attached to a motor shaft, and the thrust pads of the upper thrust bearing are not fixedly mounted to the upper bracket. Hence, trembling or vibration of the thrust pads is caused under certain conditions of the pads and the slide contact member.
It is considered that such trembling or vibration of the thrust pads is caused for the following reasons. The thrust pads hang their slide contact surfaces down on the bracket. The hydromechanics of upward thrust which is generated by the pump unit and is given to the motor shaft varies, and the weights of the rotor of the electric motor, the impeller and the like, mounted to the motor shaft, are applied to the motor shaft as a downward thrust. Therefore, the varying hydromechanic upward thrust applied to the shaft is always subtracted by the downward thrust applied by the weights of the members mounted to the shaft, and accordingly the variation rate of the upward thrust applied to the shaft is enlarged. When the upward thrust is small, the motor shaft is floating, and when the upward thrust is large, the contact pressure between the thrust pads of the upper thrust bearing and the slide contact member of the motor shaft becomes large, thereby enlarging the variation rate of the upward thrust applied to the motor shaft.